Semiconductor substrates (wafers) are generally flat and, during processing (e.g. etching, vapor deposition), are fully supported on a generally flat surface of a pedestal or susceptor. During processing, the wafer is exposed to elevated temperatures and magnetic and electrical fields. A typical processing chamber 20 is shown in FIG. 1. A gas distribution plate 33 directs gas toward a wafer 35 supported by an anodized aluminum susceptor 27. The gas distribution plate 33 is supplied with alternating RF current which causes the gas to form a plasma. The susceptor 27 is grounded to aid in the deposition process and stabilize the plasma envelope.
The electrical fields can cause an electrostatic charge to form on the wafer. The charge on the wafer attracts an opposite charge in the conductive metal susceptor to create an electrostatic attraction between the wafer and the susceptor which results in a sticking force preventing the wafer and susceptor from separating. This sticking force can and does cause problems when the wafer is being handled and transferred into and out of a wafer processing chamber.
The small gap between the wafer and the flat susceptor, and the insulating properties of the surface of the susceptor material, e.g., the anodized material at the surface of an anodized aluminum susceptor, constitute a dielectric layer between the wafer and the susceptor. This dielectric layer causes the susceptor and wafer to a act as two plates of a capacitor. The sticking force between the wafer and susceptor can therefore be modeled by capacitor principles.
The electrostatic attractive force (sticking force) between the wafer and susceptor is directly proportional to the dielectric constant of the anodization coating and is inversely proportional to the square of the distance between the surfaces of the two pieces, i.e., the thickness of the anodization coating on the pedestal.
This sticking problem is illustrated in FIGS. 1 and 2. A semiconductor wafer 35 is shown in a semiconductor processing chamber 20. A plurality of lift fingers 24 (usually four) are situated to pass through lift pin holes 30 near the perimeter of the susceptor 27. The lift fingers raise the wafer above the pedestal when processing of the wafer is completed and the wafer is to be removed from the chamber by a robot (not shown).
Specifically, relative motion between the lift fingers and the susceptor (the lift fingers rising and/or the susceptor descending) causes the lift fingers 24 to come in contact with a bottom side of the wafer 35 and exert a force on the bottom of the wafer to start to lift the wafer 35 from the susceptor 27. The electrostatic sticking force attracting the wafer 35 to the susceptor 27 must be overcome to lift the wafer clear of the surface of the susceptor. The electrostatic attraction force between the wafer and the pedestal can hold the center of the wafer against the pedestal while the lift fingers begin lifting the perimeter of the wafer. This flexes the wafer into a concave shape (bowl shape) as shown in FIG. 1. As the lift fingers continue to rise through the susceptor, the force the lift fingers exert on the wafer continues to rise and continues to increase the flexing of the wafer until the sticking force holding the wafer 35 to the susceptor 27 is overcome. The release of the sticking force is unpredictable and is often sudden.
The release is unpredictable because the sticking force is dependent on the spacing and relative positioning between the wafer and the susceptor. Each susceptor and each wafer are made as uniformly as possible, but manufacturing tolerances introduce small variations which unpredictably alter the sticking force. The adhesion can be lopsided or otherwise non-uniform resulting in a non-uniform flexing and unpredictable release of the wafer from the susceptor.
When the release of the wafer from the susceptor is gradual and smooth, the lift pins when raising the wafer from the susceptor will initially contact the wafer at lift locations on the underside of the wafer, and will maintain contact with those same lift locations throughout the lifting cycle of the lift fingers.
When the release is sudden, the flexing of the wafer causes the lift fingers to flex or change their support location on the bottom of the wafer. Because the release is sudden, the restoring momentum of the mass at the center of the previously bowl shaped wafer will cause the wafer to unweight from the lift fingers, as the wafer 35 shown in FIG. 2. In extreme cases the momentum will be sufficient to launch the wafer off the lift fingers (as pictured in FIG. 2 by the dashed lines 35a, depicting such a launching). The wafer after reaching the peak of its travel will return to again be supported by the lift fingers. The unweighting or launching of the wafer is not generally uniform and can and does cause oscillation and sliding or relative movement between the wafer and the lift fingers.
In these scenarios, the initial points of lift finger contact on the bottom of the wafer are not maintained when the wafer is suddenly released from the susceptor. The movement of the wafer relative to the ends of the lift fingers causes the initial alignment and registration of the wafer to be altered. This alteration must be recognized and corrected before future detailed processing steps can take place. In some instances the displacement of the wafer from its original position on the lift fingers is so great that the wafer can no longer be handled by the normal wafer handling robot blade, and normal processing must be interrupted until the displaced wafer's position can be corrected so that further processing and/or handling can continue.
The regular displacement of wafers from their registered positions during processing is a big problem in semiconductor processing. It creates delays in production and can ruin whole wafers when left uncorrected in subsequent processing steps.